The present invention relates to a plasma process chamber for processing semiconductor substrates.
A plasma process chamber is used in semiconductor fabrication processes, for plasma enhanced chemical vapor deposition (CVD), reactive ion-etching (RIE), and ion implantation. FIG. 1 shows a conventional process chamber 20 having a gas distributor 22 that provides process gas to the chamber. A coil power supply 24 powers an inductor coil 26 adjacent to the chamber that inductively couples RF energy to the process gas to form a plasma. Process electrodes that are used to couple RF power to the plasma typically include a cathode 28 below the substrate and an anode 32 surrounding the cathode. The cathode 28 is electrically insulated from the anode 32 by one or more quartz or silicon dioxide insulator shields 34 that extend below and around the cathode. The power supply 36 applies an impedance matching RF bias power to the cathode and the anode is formed by electrically grounded sidewalls and top walls in the chamber 20. The cathode 28 is capacitively coupled to the anode 32 via the electrostatic chuck 38 that rests on the cathode, the substrate 30, and the plasma sheath that forms at the boundary of the cathode. The capacitively coupled electric field energizes and accelerates the plasma ions toward the substrate 30.
Conventional chambers 20 often have low plasma ion densities and plasma ion energy distributions which are spread out over a wide range of energy levels with multiple energy level peaks. The plasma ion density and energy levels depend upon the electron density or energy distribution in the plasma sheath, the power and frequency of the capacitively-coupled RF power applied to the electrodes 28, 32, the process gas composition and pressure, and the components inside the chamber. The plasma sheath is an electron deficient region that arises due to the difference in mobility between electrons and positive ions in the plasma, and has an associated voltage waveform that is effected by the resonance, impedance load, and other electrical characteristics of the chamber. This voltage waveform is perturbed from normal by the harmonic content of the plasma sheath. The RF bias and impedance matching circuit, power transmission lines, structure of the electrodes 28, 32, and the components inside the chamber 20, all effect the development of harmonics at the plasma sheath, which in turn disturb the sheath voltage waveform over time giving rise to perturbed waveforms. The perturbed waveform can give rise to a "dual predominant" multimodal plasma energy distributions in which the plasma ion energy distribution curve has two or more peaks. Such multi-peak energy distributions are undesirable because a large spread of plasma ion energies provides lower average plasma ion energy levels that result in poor plasma performance, for example, etch stopping in processes for etching high aspect ratio trenches.
Low plasma density can also result from the large number of components that are in the chamber 20, such as the dielectric or insulator shields 34, electrostatic chuck 38, focus rings and gas seals. These components provide a large chamber impedance load due to the number of capacitive couples 40 formed across the components. For example, capacitive couples 40 extending from the sidewalls of the cathode 28, through the adjacent dielectric components, and to the surrounding anode walls 32, form individual "leaky" capacitors that add impedance loads to the chamber. As a result of these parasitic or stray capacitances, it is difficult to tune the RF bias power source to match the plasma impedance, because the impedance load of the stray capacitances is typically much larger than the impedance load of the plasma. Also, the stray capacitances can cause the plasma sheath waveform to change as a function of the magnitude of the capacitive effects. Thus it is desirable to have a chamber that minimizes stray capacitance and impedance loads between the cathode, dielectric structures, and other components in the chamber, to provide more stable and controllable plasma characteristics.
Components which capacitive couple with one another in the chamber can also significantly weaken the electric field in the chamber resulting in low vector directional energies of the plasma ions. The low directional energy of the plasma ions at least partially results from capacitive coupled surfaces which are at angles other than the perpendicular direction to the plane of the substrate. In particular, coupling at the vertical peripheral edges of the cathode 28 can cause electric field components that are perpendicular to the plane of the substrate 30 to deviate from normal and bend toward the peripheral edge of the cathode 28. Also, the electrostatic chuck 38 that is used to securely hold the substrate in the chamber provides additional interfaces that capacitive couple to one another. Plasma species are also attracted toward the capacitive couples formed across the insulator shield 34 surrounding the substrate 30. These capacitive couples result in asymmetric distributions of plasma ion energy levels or plasma ion densities across the substrate surface causing the peripheral edge and center of the substrate to be processed at different rates.
Another problem with conventional chambers arises from use of the quartz insulator shields 34 surrounding the cathode 28. The insulator shields 34 reduce the ratio of the surface area of the cathode 28 to the surface area of the anode 32, by occupying space in the chamber, and preventing the cathode from extending across the entire width of the chamber. A low cathode to anode surface area ratio reduces the plasma ion density and energy in the chamber. Also, the smaller diameter of the cathode can reduce plasma ion density or energy levels at the peripheral edge of the substrate 30 as compared to the center of the substrate. Moreover, the quartz insulator shield 34 has a complex shape, is expensive to fabricate, and often requires replacement due to erosion in the corrosive plasma environments. The insulator shield 34 also acts as a thermal insulator surrounding the cathode 28 which reduces the flow of heat to and from the substrate. Thus, it is desirable to have a plasma process chamber that is absent insulator shields 34 around the cathode 28, and that allows the cathode 28 to be extended across substantially the entire width of the chamber. However, this is not possible in present chamber designs, because the cathode 28 typically comprises a metal base plate that is exposed to the plasma and would electrically short in the plasma, without the presence of the insulator shield 34 around the cathode.
Conventional chambers also have problems that arise from the arrangement of the inductor coils 26 adjacent to the chamber 20. Inductor coils 26 that are parallel to the sidewalls of the chamber 20 provide non-uniform fields across the substrate surface with strong inductive electric fields at the center of the substrate 30 and weak inductive fields at the peripheral edge of the substrate 30. On the other hand, inductor coils that inductively couple energy through flat dielectric ceilings which allow RF inductive electric fields to permeate therethrough (not shown), do not allow capacitive coupling of energy through the ceiling because it is made of non-conducting dielectric material. It is desirable for both the capacitive and inductive electric field components in the chamber to have highly directional vector field components that are substantially perpendicular to the surface of the substrate, and which extend uniformly across the entire substrate surface.
Another problem with conventional designs for inductor coils 26 arises from the relatively large volume of space that is required in a chamber 20 to form an inductively coupled plasma from conventional inductor coils. The inductor coil 26 typically encircles the chamber 20, and the chamber has to have a sufficiently large volume to provide a large skin depth for the RF induction field from the coil. Without the large skin depth, the magnitude of the RF induction field formed in the chamber 20 would not be sufficiently high to generate a plasma. However, a plasma chamber 20 having a large internal volume is undesirable because relatively large power levels of RF bias voltages must be used to energize the plasma ions, causing excessive heating of the substrate. Also, it is more difficult to stabilize or maintain uniform plasma ion densities and energy distributions across large plasma volumes. It is also more difficult to finely tune a high density plasma that occupies a large volume of space because of the increased energy perturbations that occur in the larger plasma volume. Novel induction coil designs are needed to provide high density plasmas in smaller volume chambers that have diameters slightly larger than the diameter of the substrate 30.
Thus there is a need for a plasma process chamber that provides a high density plasma with a uniform energy distribution and reduced perturbations of ion energy distribution. There is also a need for an apparatus that provides a uniform distribution of plasma ions having high directional energy vectors, across the entire surface of the substrate. There is also a need for process electrode and inductor coil designs that can generate a stabilized and controllable plasma in the chamber. There is a further need for a plasma processing chamber that eliminates use of insulator shields around the cathode and that provides a high anode to cathode surface area ratio in the chamber.